Ethnicity was not important in the Medieval world. Common people did not have to communicate with the state. They were supposted to work and pay taxes. The best they could hope for was to be left alone.

In the 19th century that changed.

The flashpoints in the Early Byzantine Empire were religious and doctrinal, but those often ended up being closely correlated with ethnicity—particularly with dyophysitism vs monophysitism (to use each side’s pejoratives). The bulk of the peoples lost by the Empire to the Caliphate were not native speakers of Greek, after all.

After Chalcedonian Christianity, “heresies” remained a flashpoint, but you do also start seeing more clearly ethnic-based conflict. I don’t know what else to call the Uprising of Asen and Peter, for instance:

In fact, the victorious brothers raised a church to the same St Demetrius whose cult site was in Salonica; in other words, they asserted religious continuity with the Empire, but not political allegiance:

After their return, many of the protesters were unwilling to join the rebellion. The brothers Peter and Asen built the Church of St Demetrius of Thessaloniki in Tarnovo, dedicated to Saint Demetrius, who was traditionally considered a patron of the Byzantine city of Thessaloniki, and claimed that the Saint had ceased to favour the Byzantines: “God had decided to free the Bulgarians and the Vlach people and to lift the yoke that they had borne for so long”.